1. Field of the Invention
This invention relates to a radio frequency coil, and more particularly to a radio frequency coil for use in a nuclear magnetic resonance imaging apparatus for medical diagnosis.
2. Discussion of Background
A radio frequency coil for nuclear magnetic resonance imaging is in effect a radio frequency antenna which is used to generate a radio frequency magnetic field for exciting the nuclei in a sample, as well as to receive the radio frequency signals which are emitted by the excited nuclei. FIG. 1 illustrates a conventional radio frequency coil of the type described in the Journal of Magnetic Resonance, Volume 36, pages 447-451 (1979). The illustrated coil comprises a pair of H-shaped members 1 which are made of plates of an electrically conducting material such as copper. The H-shaped members 1 are symmetrically disposed about a longitudinal axis so as to form a hollow, cylindrical tube. Each H-shaped member 1 has a longitudinally-extending web 1a, which is referred to as a vertical band, and two laterally-extending flanges 1b at opposite end of the web 1a, the flanges 1b being referred to as wings. Adjacent flanges 1b are electrically connected with one another by means of chip capacitors 2. Two electrically conducting rings 3a and 3b, at least one of which is grounded, are coaxially disposed within the tube at its opposite ends. The guard rings are separated from the inner surface of the H-shaped members 1 by an unillustrated layer of an electrically insulating material such as Teflon. The guard rings shield a sample from the electric fields which are generated by the radio frequency voltage present between the wings and decrease dielectric losses within a sample. Two electrodes 4a and 4b are integrally formed on one of the H-shaped members 1 and on the bottom guard ring 3b, respectively. The two electrodes 4a and 4b serve as feeding points for the coil and are connected to an unillustrated radio frequency transceiver through an unillustrated impedance matching circuit. The transceiver is used to excite the radio frequency coil and to receive the signals which the coil picks up from a sample.
The H-shaped members 1, the chip capacitors 2, and the guard rings 3a and 3b together form a three-dimensional circuit which is equivalent to a series resonant circuit, the resonant frequency of which can be adjusted by variable capacitors in the unillustrated impedance matching circuit.
Because each of the webs 1a of this conventional radio frequency coil is a single conducting plate, the stray capacitances between the coil and its surroundings are large, making it difficult for the coil to cope with magnetic fields of higher frequencies. Furthermore, when a plurality of such coils are used in combination, the efficiency of magnetic field generation decreases due to eddy current.